1. Field of the Invention
This invention relates to a film comprising poly(m-phenylene isophthalamide) having especially superior properties as an electric insulating material, and a process for its production.
2. Description of the Prior Art
It is known that poly(-m-phenylene isophthalamide) and its copolymers have superior thermal stability, and the development of various fabricated articles using such polymers, for example, fibers or films, has been under way.
As regards such films, U.S. Pat. Nos. 3,006,899 and 3,063,966 disclose that films prepared from poly(m-phenylene isophthalamide) and its copolymers are useful as thermally stable insulating materials. These patents also disclose a process for producing films which comprises casting a solution of the above polymer in dimethyl formamide containing lithium chloride into film form, heating the cast film in a hot oven to evaporate off the solvent, immersing the resulting film in hot water to remove the remaining solvent and the salts, and then drying the film in vacuum to obtain a final film product, and a process for obtaining a biaxially oriented film by heating the above cast film in a hot oven to evaporate off the solvent, and subjecting the resultant film to a hot rolling in two directions.
U.S. Pat. No. 3,094,511 discloses a process for obtaining a biaxially oriented film which comprises dissolving a copolymer obtained by reacting 70 mol% of isophthaloyl chloride and 30 mol% of terephthaloyl chloride with m-phenylenediamine in dimethyl formamide, casting the resultant solution into film form, washing the cast film with water, drying it, and stretching it at an elevated temperature under steam pressure using a two-way stretcher.
U.S. Pat. No. 3,354,127 discusses the mechanical properties of hot-stretched films composed of poly(m-phenylene isophthalamide) or its copolymers.
Furthermore, U.S. Pat. No. 3,696,076 discloses a process for producing films which comprises subjecting a cast film prepared from a similar polymer solution, i.e., a solution of a copolymer prepared by reacting a mixture of isophthaloyl chloride and phthaloyl chloride (molar ratio: 70:30) with methaphenylenediamine, to a "stage-curing" wherein the curing temperature is progressively elevated from about 130.degree. C to a temperature above 200.degree. C.
With these conventional techniques, it is difficult to produce aromatic polyamide films having especially superior properties as an electric insulating material.
Poly(m-phenylene isophthalamide) cannot be melt-shaped because of its infusibility, and, therefore, it is generally fabricated by a wet shaping process or a dry shaping process from a dope in an amide type solvent such as those described hereinbelow. Film preparation by a wet process or a dry process, however, possesses economic and technical disadvantages, and results in films having unsatisfactory properties.
For example, in a wet process the choice of a coagulating agent is in itself a problem, and even when special consideration is given, for example, to the coagulating ability of the coagulating agent, the coagulating time, and the coagulating temperature, the resulting film is opaque, and films having superior mechanical and electrical properties cannot be obtained.
The dry process involves evaporating off the solvent in a heated atmosphere. However, since the amide type solvent generally used has a high boiling point and a high latent heat of evaporation, the drying of the film requires high temperatures and long times. In other words, very long times are required in order to obtain a solvent-free film product by evaporating off the solvent in a cast film prepared from a solution comprising an aromatic polyamide and an amide type solvent by drying or curing alone. Furthermore, unstretched films obtained after mere solvent removal have only unsatisfactory electrical and mechanical properties as an electric insulating material. When the solvent is removed by drying the cast film, evaporation of the solvent from the film proceeds relatively rapidly until the residual solvent content of the film reaches about 20% by weight, but further evaporation of the solvent becomes extremely difficult. For example, when the film is treated at a temperature of as high as above 200.degree. C in an attempt to obtain a film having a thickness of 25 microns, drying for more than 10 hours is required in order to reduce the residual solvent content to less than 5% by weight.
When a solution containing inorganic salts is used, the inorganic salts have a strong affinity for the amide type solvent and form soluble complexes which inhibit the evaporation of the solvent. For this reason, it is practically impossible to evaporate and remove all of the solvent only by drying or curing.
Accordingly, in order to obtain solvent-free product films, it is necessary to first evaporate off the solvent to some extent by drying, and then wash the film with water to remove the remaining solvent and/or inorganic salts. Various investigations we made in this regard, however, led to the discovery that with a conventional washing with hot water, the resulting films do not possess especially superior properties as an electrical insulating material. Films obtained by a conventional washing in hot water do not possess superior properties as an electric insulating material, not only in the unstretched state, but also in the stretched state attained after the application of a conventional stretching technique such as hot rolling, stretching at elevated temperatures under steam pressure, or a hot stretching treatment.
Transparent aromatic polyamide films, therefore, do not exist in the market, and only insulating papers formed by paper forming techniques from fibers and fibrid particles are now available to some extent as an electrical insulating material. With such insulating papers, however, superior insulating performance cannot be obtained.
The properties of films as an electric insulating material can be shown by their dielectric strength. The "dielectric strength", as preferred to in the present application, is measured by the method described in ASTM D-149-64.